add syntax for mixtures of distributions

Andrew suggested something to make mixture modeling easier. I'm not sure what he's thinking in terms of syntax, but maybe something like a distribution like this:

y ~ finite_mixture(lambda, 
                   normal(mu1,sigma1), 
                   normal(mu2,sigma2), 
                   normal(mu3,sigma3) );

or a special "function" like

target += finite_mixture(lambda,
                         normal_log(y,mu1,sigma1),
                         normal_log(y,mu2,sigma2),
                         normal_log(y,mu3,sigma3));

Of course, these aren't functions with the usual type of signature.

Another issue is how to deal with vectorization.

Review the proposed finite_mixture syntax and the issue's concern about nonstandard function signatures and vectorization. First resolve the intended syntax and typing behavior; done means the mixture form and its vectorization rules are specified clearly enough to implement.